In the field of plastics processing, thermoforming is a recognised method for the mass production of lids, vessels and bowls used as packaging.
Sheeting for this process for food is typically between 150 and 3,000 μm thick. For technical components (automotive, household, etc.), sheet thicknesses of up to 15 mm are also common.
The sheeting suitable for the thermoforming process is produced on polishing stack systems. Generally, polishing stacks with three rolls are used for this purpose and, in the case thereof, one or two adjustable nips are produced using various mechanical roll feed concepts. Coming from a slit die, the plastics material is sized in the polishing stack, and the surface finish of the sheeting is produced. The melt emerging from the extruder is sized and cooled down. Post-cooling rolls are sometimes placed downstream in order for the required end temperature to be reached.
The above-described prior art is limited in terms of output, since there is a maximum of two nips with which to size the sheeting surface. If, however, the operational capacity is increased, it can then be seen that the already sized sheeting surface melts again, triggered by the core heat of the sheeting, and the surface formation, which is designed so as to be high gloss but also to have an embossed structure in some regions, is destroyed. This shortcoming becomes greater with increasing sheeting thickness. The output limit is determined by the first two nips and the cooling properties of the two to three cooling rolls associated therewith.
Attempts have been made to remedy these limitations using various methods:                Dual- and multiple-passage system design:        This has shortcomings since such a wide machine becomes disproportionately more difficult to handle with increasing width. In addition, investment costs increase although the achievable sheeting tolerances decrease.        Use of thinner wall thicknesses for the rolls and use of materials of higher conductivity:        This has shortcomings because the mechanical instability rises with decreasing casing thickness. The higher the conductivity of materials (e.g. copper), the more vulnerable the roll surfaces are to mechanical damage.        Selection of larger roll diameters to lengthen the cooling path:        This has shortcomings because the ease of handling the machine is determined by the distance from the extrusion die to the roll and thus to the nip, yet this distance between the die and the nip widens consistently as the roll diameter becomes larger. In addition, operation at low take-off speeds is rendered more difficult, and thus so too are the start-up and switchover processes. The cooling itself is substantially less even with the long, one-sided cooling (one side steel-one side air) and the sheeting is thus less uniform.        
DE 10 2005 006 412 additionally proposes placing a cooling path of roll pairs arranged in tandem downstream of a polishing stack, it thereby being possible to cool and shape the sheeting over a longer path.